Disc drive machines record and reproduce information stored on a recording media. Typical hard disc drives, often referred to as Winchester disc drives include one or more vertically aligned, rotating information storage discs, each having at least one associated magnetic read/write head or slider that is adapted to transfer information between the disc and an external computer system. The information storage discs are journaled about a spindle motor assembly capable of rotating the discs at high speeds. The sliders are carried by a plurality of vertically-aligned, elongated flexure arms that in turn are carried by a head positioner assembly. The head positioner assembly is adapted to move the sliders back and forth in unison across the face of the vertically-aligned discs. The head positioner assembly is traditionally either rotationally mounted or takes the form of a carriage that is free to move back and forth along a single axis. In either case, the head positioner assembly is adapted to precisely position the heads relative to the magnetic information storage discs.
In the typical operation of such a disc drive, the sliders rest on landing zones on the surface of their associated discs when the power is off. In operation, the drive unit is powered up and the disc pack begins to rotate. Once the disc reaches a certain critical speed, the sliders rise slightly off the landing zone under the lifting influence of a small air bearing which is created by the speed of rotation of the discs. During normal operation, the sliders remain floating above the discs are said to "fly" over the discs, thereby preventing wear on the disc surfaces and potential destruction of the data. However, until the sliders rise, there is considerable friction associated with the heads dragging over the discs, which causes wear to both the heads and the discs. When the drive unit is powered down, the same friction will occur between the surfaces of the heads and the rotating disc surfaces. In practice, breaking the heads free from the surface of the disc on which the sliders have been resting can require a considerable force to overcome the force which holds the two smooth surfaces together (called stiction).
In order to reduce this friction and minimize damage, the discs are coated with a protective layer and lubricants are applied to the disc surfaces. Additionally, the discs typically require a dedicated landing zone where the sliders can slide to a halt and rest when the drive unit is turned off. No data can be stored in this dedicated landing zone since repetitive starting and stopping of the disc tends to wear the landing zone. Consequently, the amount of data that can be stored on each disc is reduced. Moreover, a relatively large disc drive motor is required to overcome the adverse friction and stiction effects. Further, a motor brake is often also required to stop the rotation of the discs when the motor is turned off to reduce frictional wear.
Thus, it should be apparent that it is desireable to raise the sliders during power-up and to keep them up for a brief period after power-down until the disc pack has essentially come to a halt. Such an ability would eliminate the adverse effects of friction.
A wide variety of arrangements have been proposed to raise and lower the sliders at selected times. A few attempts have been made to take advantage of the shape memory metal phenomenon within the lifter mechanism. The phenomenon of shape memory is, of course, well understood. It is based on the thermoelastic martensitic transformation which is briefly explained hereunder. A shape memory alloy, such as Ti-Ni alloy, has a high temperature austenitic phase wherein the crystal structure is body center cubic. When cooled below its transformation temperature, the austenitic structure undergoes a diffusionless shear transform into a highly twinned martensitic crystal structure. In the martensitic phase, the alloy is easily deformed by the application of a small external force. However, in the austenitic phase, the alloy is very strong and is not easily deformed. When the alloy is heated through its transformation temperature, the martensitic phase is elastically returned to the austenitic phase (referred to as an inverse transformation) according to a given ordered crystal and orientation law. A notable characteristic of the alloy is the extremely large recovery force that is generated when returning to the austenitic phase. Therefore, the employment of a resilient force as a bias force capable of deforming the alloy in its martensitic phase permits the alloy to be used as a reversible actuator with temperature cycling. Further, since the recovery force which is generated with the return of the austenitic phase is quite large, it is possible to take advantage of the recovery force to do work.
It is important to the design of slider lifters to provide a device which raises and lowers the sliders without requiring significant modification of the design of the flexure arms. Avoiding substantial modification of the existing design of the flexure arms is a primary requirement of a successful device for loading and unloading the head. The lack of commercial success of prior-art designs is due in significant part to their attempts to seriously alter the flexure design. The reason is that in a conventional floating head slider, the head is expected to float in a stable fashion about 0.2 micrometers over the surface of the disc which is rotating at a constant speed. Therefore, the combined flexure arm and slider are very sensitive to their loading and air foil characteristics. Indeed, a great deal of time and effort has gone into their design. Accordingly, any design that requires modification of the flexure arm is looked upon with great disfavor. Another drawback of prior art designs is that they tend to require the assembly of a relatively large number of intricate parts. Such designs are disfavored since they add significantly to the costs of manufacturing and increase the probability of defective parts.
Two prior art patents representative of the use of shape memory alloy technology in lifter arrangement for magnetic disc drive apparatus are U.S. Pat. No. 4,605,979 (to Inoue) and 4,684,913 (to Yaeger). The devices described in both of these patents incorporate the use of shape memory alloys to attempt to provide a more efficient system for loading and unloading a transducer head from a disc surface. However, they both utilize relatively high profile devices which require significant spacing between adjacent discs. In present disc drive technology, such spacing is simply no longer available. To maximize the capacity of disc drives being manufactured, discs are spaced as closely as the supporting flexure will allow. In view of the foregoing, it should be apparent that it is important that the lifter design maintains an extremely low profile without requiring modification of the flexures' design.
The '979 discloses several potential designs for lifting the flexure to unload the slider. Each of the disclosed designs utilizes a resilient biasing member in combination with the shape memory alloy to load and unload the slider. In the sixth figure, a wire formed of shape memory metal extends forward from the guide bar upon which a load arm (flexure) is mounted. The wire is coated with a silicone resin which serves as the resilient biasing member. The silicone resin also serves to thermally insulate the wire. However, such an arrangement has the drawback of requiring a delicate coating of the shape memory wires in their martensitic state which adds to the complexity and costs of assembly.